The present invention relates to spheroidal graphite cast iron whose matrix is composed of a mixture of bainite and retained austenite, and a method of producing spheroidal graphite cast iron in which after a machining process has been performed, an austempering process for subjecting the casting to austenitizing and then, isothermal transformation is performed.
It is well known that in order to improve such mechanical properties as strength, toughness, fatigue strength, etc. of spheroidal graphite cast iron, it is remarkably effective therefor to change its matrix into a mixture of bainite and retained austenite by austempering. However, in this case, such a problem arises that since retained austenite in a state of lumps is precipitated by austempering, this retained austenite is changed into martensite during a machining process through its transformation induced by the machining process, thereby resulting in very poor machinability.
In order to obviate this problem of very poor machinability, it has also been conventionally so arranged that the machining process is performed prior to the austempering process. However, at the time of this machining, since the casting as cast has a large hardness, the casting is required to be initially annealed as primary heat treatment so as to have a ferritic matrix and then, is machined. Furthermore, in the case where after a casting has been machined, the casting is subjected to austempering, strain of the casting is produced at the time of a heating step and a cooling step of the austempering process. Therefore, in the case of a product requiring high accuracy, the product should be, after having been subjected to austempering, finished by machining or grinding. Meanwhile, as described above, since machinability of the casting deteriorates if the casting is subjected to austempering, finishing of the casting is quite troublesome, thus resulting in deterioration of its productivity and increase in production cost.
Therefore, if machinability of the casting subjected to austempering could be improved, it will be possible to raise productivity of the casting during the above described finishing process and reduce the production cost. For example, Japanese Patent Laid-Open Publication (unexamined) No. 149428/1986 proposes spheroidal graphite cast iron in which molten metal is rapidly cooled and solidified by using a metal mold or a mold provided partially with a chiller such that the number of graphite grains in the casting is increased, thereby improving machinability of the casting subjected to austempering. In this known casting method employing the metal mold or the mold provided partially with the chiller, it is surely possible to obtain the casting containing a great number of graphite grains. However, this known casting method has such a drawback that since chill is likely to be precipitated, addition of alloy elements, which is performed so as to obtain stable bainite structure through improvement of hardenability in isothermal transformation mainly for the purpose of improving strength of the casting, is restricted.
Meanwhile, in the case where spheroidal graphite cast iron is produced, there is a possibility that precipitation of chill having an excessively ill effect upon machinability and toughness of the casting occurs according to casting methods and casting conditions. When precipitation of chill occurs, the number of graphite grains usually decreases, so that segregation of alloy elements, precipitation of carbides having a large hardness, etc. are caused, thereby further hampering machinability of the casting.
As a countermeasure against deterioration of machinability due to precipitation of chill, there has been proposed so-called two-stepped annealing in which, in a first step, the casting is heated and held at a predetermined temperature higher than the A.sub.1 transformation point for a predetermined time period prior to annealing in primary heat treatment performed after casting and prior to machining such that chill is decomposed and then, in a second step, the casting is heated and held at a predetermined temperature lower than the A.sub.1 transformation point for a predetermined time period so as to be annealed. However, this prior method has such disadvantages that cost of heat treatment becomes high and a long time period is required for performing heat treatment.
Meanwhile, during production of spheroidal graphite cast iron, if graphitizing is promoted by a proper method such that the number of graphite grains per unit area of the casting is increased, a number of graphite grains having ferritic structure provided therearound are scattered in its matrix, so that precipitation of chill can be restricted and change of structure to ferrite can be performed relatively easily.
Meanwhile, during production of spheroidal graphite cast iron, inoculation is generally performed for the purpose of preventing occurrence of chill, promoting graphitizing, etc. Conventionally, in inoculation, a ladle inoculation method is employed in which inoculant is added to molten metal in a ladle before the molten metal is poured from the ladle into a mold. In this ladle inoculation method, since a time period from inoculation to solidification of the molten metal becomes long, effects of inoculation are lessened and thus, there is a possibility of precipitation of chill having an excessively ill effect upon machinability and toughness of the casting.
Therefore, it will be understood that if the ladle inoculation method is replaced by a pouring inoculation method in which inoculant is added to pouring molten metal which is being poured from the ladle into the mold, or an in-mold inoculation method in which inoculant is added to molten metal by filling the inoculant into a chamber formed in the course of a runner, the time period from inoculation to solidification of the molten metal is reduced, so that effects of inoculation are enhanced and thus, the number of graphite grains in the casting is increased, thereby resulting in restriction of precipitation of chill.